In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of an aqueous mixture, for example, comprising water and one or more organic materials such as a monohydric alcohol, or a polyhydric alcohol.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer (IRL). There are generally two types of IRLs. The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling, which non-porous coating absorbs ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. Typically, the support is a smooth resin-coated paper and the coating is optically transparent and very smooth, leading to a very high gloss “photo-grade” inkjet recording element. However, this type of IRL usually tends to absorb the ink slowly and, consequently, the imaged receiver or print is not instantaneously dry to the touch.
The second type of ink-receiving layer or IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open interconnected pores of the IRL, substantially by capillary action, to obtain a print that is instantaneously dry to the touch. Typically the total interconnected inter-particle pore volume of porous media, which may include one or more layers, is more than sufficient to hold all the applied ink forming the image.
Basically, organic and/or inorganic particles in a porous layer form pores by the spacing between the particles. The binder is used to hold the particles together. However, to maintain a high pore volume, it is desirable that the amount of binder is limited. Too much binder would start to fill the pores between the particles or beads, which would reduce ink absorption. On the other hand, too little binder may be insufficient to prevent cracking of the porous layer.
A porous inkjet recording medium that is glossy usually contains at least two layers in addition to the support: a base layer nearer to the support, and a glossy image-receiving layer further from the support. One method of obtaining a “photographic-grade” gloss is to coat the inkjet receiving layers on a resin-coated paper support. Resin-coated paper support is relatively costly, however, and requires an extra resin-coating step in its manufacture.
For example, Bermel et al., U.S. Pat. No. 6,630,212, describes an inkjet recording medium comprising two porous layers coated on a resin-coated support paper. The two layers are coated simultaneously by a pre-metering method, extrusion hopper coating, on a polyethylene resin-coated support paper. The base-layer coating composition comprises filmed alumina particles, PVA binder, and coating aids at a solids content of 30%. The coated weight of the base layer is 43 g/m2. An image-receiving layer over the base layer comprises fumed alumina particles, cationic polymeric latex dispersion, and poly(vinyl alcohol) (PVA) binder. The coated weight of the IRL is 2.2 g/m2. Alumina is a relatively expensive material for recording materials of high ink capacity.
Inkjet recording media with “photographic-grade” gloss can also be made when coating on a plain paper support. Because plain paper supports are generally rougher or less smooth than resin-coated paper supports, however, it is typically necessary to use special coating processes, such as cast coating or film transfer coating in order to achieve a smooth, glossy surface on the image receiving layer. These specialized coating methods are constrained in their productivity by drying considerations or by extra steps. Mild calendering with heat and pressure may also be used in combination with conventional post-metered (blade, rod, or air-knife) or pre-metered (bead or curtain) coating processes on plain paper in order to produce a glossy surface on the image-receiving layer. Excessive calendering may result in a loss of ink absorbing capacity.
Manufacturing processes for porous inkjet receivers typically employ coating of aqueous particle dispersions. Particles useful in such compositions generally possess a surface charge that aids the stability of the dispersion by providing repulsive forces between particles and attractive forces with the polar molecules of the aqueous phase. These particles may be characterized according to the chemical nature of the surface. If the charged chemical moieties on the particle surface predominately possess a formal negative charge, the particle is herein defined as an anionic particle. Dispersions of calcium carbonate and silicon oxide particles in their natural state (at moderate pH range between 3 and 10) are examples of anionic particles. In contrast, dispersed particles with net positive surface charge are termed herein cationic particles. Alumina is an example of a cationic particle often used in porous layers of inkjet receivers.
Inkjet receivers with porous layers employing the aforementioned particles are known. Schultz, et al., in US Patent Publication No. 2007/0202279, describe a porous three-layer ink-receiving material coated on plain paper support. The porous base layer comprises anionic pigments, for example, precipitated calcium carbonate (PCC) and silica gel, and binders, for example, poly(vinyl alcohol) and styrene-butadiene latex, and a total dry weight of at least 25 g/m2. One of the main functions of the base layer in a three-layer material is to provide a smoother substrate than a raw paper upon which to coat the upper layers. In addition, the porous base layer may provide a sump for the ink fluids in the ink applied to the uppermost layer by the printer. Schultz, et al. describe a porous intermediate layer present in an amount of at least 25 g/m2 comprising colloidal alumina and a porous top layer comprising alumina in an amount of at least 1 g/m2. The base layer is coated by a post-metering method, e.g. rod coating, followed by drying and then the upper two layers are coated simultaneously by a pre-metering method, e.g. curtain coating. The material is calendered at least once, optionally at any time after the initial base-layer coating, to provide a 20-degree gloss of at least 15 Gardner units in its unprinted state.
The inkjet recording element disclosed by Schultz et al., while providing good image quality and gloss, requires a drying step between the coating of the base layer and the two upper layers because the coating compositions for the base and upper layers, respectively, comprise particles of opposite surface charge which are not compatible. The coating of non-compatible coating compositions, either simultaneously or wet-on-wet, results in coagulation of the coating dispersions, either preventing coating altogether or resulting in low gloss.
As the quality and density of inkjet images increases, so does the amount of ink applied to the inkjet recording element (also referred to as the “receiver”). For this reason, it is important to provide sufficient void capacity in the medium to prevent puddling or coalescence and inter-color bleed. At the same time, print speeds are increasing in order to provide convenience to the user. Thus, not only is sufficient capacity required to accommodate the increased amount of ink, but in addition, the medium must be able to handle increasingly greater ink flux in terms of ink volume/unit area/unit time.
The element of Schultz, et al. is capable of absorbing a high ink flux without coalescence and of providing a desirable level of gloss. The base-layer coating composition containing calcium carbonate is not compatible with the upper-layer coating compositions containing alumina. Simultaneous coating of calcium carbonate-containing compositions with alumina-containing compositions is precluded by the tendency of incompatible compositions to foul the coating apparatus as they make contact. Furthermore, the total amount of water necessary to coat a dry solids load of greater than 50 g/m2 presents a problem to dry with satisfactory results. Harsh drying conditions lead to cracking problems, but gentler drying conditions are less productive.
Chen et al. in U.S. Pat. No. 6,150,289 describe a matte surface inkjet receiver comprising a plain paper support with a coated layer of clay particles treated with a cationic polymer to render the surface charge of the particles positive. Seventy percent of the particles have an equivalent spherical diameter greater than 0.5 micron. They do not suggest a means of preparing a glossy inkjet receiver using this coating composition.
In view of the above, the production of high quality, high capacity, high gloss porous inkjet receiver materials has been complicated by multilayer structures, high coated weights of one or more layers, and relatively expensive materials or complicated manufacturing processes.
There remains an unfulfilled need for a photographic quality inkjet receiving material that is manufacturable using low-cost materials in an efficient process requiring only a single coating and drying step and that gives images with excellent gloss, color density, and image quality.